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Laurence Madden, a professor of plant pathology, and his students at the Ohio Agricultural Research and Development Center (OARDC) in Wooster, Ohio, are field-testing a commercial sensor they helped develop to measure the energy of raindrops.

Early results show that the sensor could help explain how rain spreads the fungal spores that cause some crop diseases. One day, weather stations in disease-prone areas may be able to use the sensor to forecast possible outbreaks. This work appeared in a recent issue of the journal Phytopathology.

Madden said that farmers have no way of knowing whether a particular rain shower is the kind that will spread water-borne plant pathogens such as disease-causing fungi.

“With no forecast system to warn of outbreaks, growers have to spray anti-fungal pesticides regularly just to play it safe,so they waste a lot of pesticide,” said Madden. “If we could determine whether conditions were favorable for the spread of plant pathogens, growers could make informed decisions on when to use chemicals.”

Whether a rain shower will spread these fungi is not a simple question of the amount of rain or duration of the shower, but rather a complex interaction of these factors with temperature, wind, humidity, and the size of the raindrops that fall, explained Madden.

“Large raindrops transfer more energy when they hit a surface, so they are much more likely than small raindrops to dislodge spores from an infected plant and spread them around,” said Madden. “Two rain showers may deposit the same amount of water, but if one contains larger raindrops than the other, it will be much more likely to promote the spread of water-borne crop diseases.”

If scientists had an easy way of detecting raindrop size, they could simply calculate the kinetic energy. The problem: standard rain gauges measure how much rain falls over time, but cannot record droplet size. Madden said the few sensors on the market today that do detect droplet size are both difficult to use and prohibitively expensive, costing as much as $30,000.

“We wanted an alternative way of determining kinetic energy of rainfall. We needed a sensor that worked under natural conditions where we have drops of many different sizes falling at the same time,” said Madden.

That’s why he and his students have been working with the Sensit Company of Portland, ND, to change the company’s soil erosion sensor into one that detects rainfall kinetic energy. The sensor features a two-inch disk of piezoelectric crystal that issues an electrical signal when raindrops land on it.

The researchers used a rainfall simulator to generate raindrops of different sizes, and recorded the sensor’s output for each size. Most raindrops measure between 1/100 in. and 1/5 in. wide, and the simulator can create any drop size in that range.

They then used that data to formulate equations that link the output of the sensor to the kinetic energy of the drops. The sensor data matched theoretical calculations for kinetic energy except in cases of very light rainfall.

Next, the researchers placed the sensor in a field at OARDC and took the same measurements for 85 natural rainfalls that occurred throughout 1996 and 1997.

The sensor revealed that a typical rain shower contains moments of high and low intensity rainfall. Moreover, it showed that both high and low intensity rainfalls varied widely in the kinetic energy of their raindrops. Madden said this supports the idea that the energy of rainfall should be measured by sensors, not estimated.

In the future, Madden and his students will use the sensor in their studies of the water-borne fruit rot disease anthracnose. Anthracnose commonly kills fruits such as strawberries -- the focus of Madden’s work -- as well as raspberries, blackberries, and tomatoes.

“From our laboratory work, we’ve learned some of the physical properties that determine how rain disperses spores. But even after years of study, we still don’t have a simple rule to follow -- we can’t say whether rainfall of a particular energy will definitely lead to an outbreak. The equations that we’ve developed here represent an important first step,” said Madden.

Researchers will likely be the only ones to use the sensor until they understand more about how rainfall relates to the spread of plant pathogens. After that, the sensor could work in weather stations alongside standard temperature and humidity sensors. Then the same agencies that issue crop advisories for insects and weeds could warn farmers about possible rain-borne pathogens.

The Sensit Company holds a patent on the sensor, and will continue to work with Madden and his students on future sensor modifications. Support for this work came from state and federal funds including a United States Department of Agriculture National Research Initiative Competitive Grant.

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